1. Field of the Invention
The present invention relates to a printing member for electrostatic photocopying, such as a printing drum or plate.
2. Description of the Prior Art
The printing members for electrostatic photocopying are used to form on copying paper a visible image pattern corresponding to a photo or light image of the pattern to be copied in the manner described below.
The photocopying process starts with electrically charging a surface of the printing member uniformly over the entire area thereof, onto which a photo or light image of the pattern to be copied is projected to form an electrostatic latennt image. Then a toner powder is applied to the surface of the printing member to develop thereon the latent image and copying paper is pressed against the surface of the printing member to print a visible image pattern on the copying paper.
There has heretofore been proposed a printing member for electrostatic photocopying which comprises a substrate having a conductive surface and a photoelectric-sensitive, electrically chargeable layer formed on the conductive surface of the substrate. The photoelectric-sensitive, electrically chargeable layer is a single layer of chalcogen such as selenium, or chalcogenide such as a selenium-cadmium or selenium-arsenic alloy.
With the conventional printing member of such an arrangement, the surface of the photoelectric-sensitive, electrically chargeable layer serves as the printing surface. Since this layer has a single-layer structure made of the abovesaid material, the surface resistance of the printing member is relatively small. Consequently, the printing surface is not sufficiently charged and a nonnegligible amount of charges leaks from the printing surface.
Accordingly, the prior art printing member is defective in that the visible image pattern printed on the copying paper is poor in contrast and in SN ratio.
Further, in the conventional printing member the electrically chargeable layer serves as the printing surface and has the single-layer structure as described above and, consequently, there is not produced in the electrically chargeable layer such a built-in-potential by which electrical carriers created by incident light are directed to the conductive surface of the substrate. Therefore, the electrostatic charge image cannot effectively be formed on the printing surface. The reason is as follows: The electrostatic charge image is obtained by the mechanism that charges on the printing surface at those areas irradiated by light are neutralized by electrical carriers (for example, electrons) created by light irradiation in the electrically chargeable layer, whereas other electrical carriers (holes) are discharged to the conductive surface of the substrate. Accordingly, for the formation of the electrostatic charge image it is desirable that the electrical carriers (holes) developed by the light irradiation in the electrically chargeable layer be rapidly released to the conductive surface of the substrate. Since the electrically chargeable layer of the conventional printing member is not of the structure that develops therein the aforementioned built-in-potential, however, the electrical carriers (holes) are not quickly discharged to the conductive surface of the substrate.
In consequence, the printing member employed in the past has the drawbacks that the visible image printed on the copying paper is poor in contrast and small in SN ratio.
Moreover, the prior art printing member is relatively small in the wear-resistance of the printing surface because the electrically chargeable layer acts as the printing surface. Hence it has a relatively short lifetime.
Besides, the aforesaid material used for the electrically chargeable layer is poisonous and cancer-developing; therefore, the fabrication of the conventional printing member involves danger and care should be taken of in the handling of the printing member itself and the copying paper with the visible image printed thereon.